The Mahomet Aquifer is
the remnant of the prehistoric Mahomet river valley. Approximately 1.6
million years ago, when our evolutionary ancestors were beginning to
walk upright and use stone tools, the valley in which the aquifer lies
was a large stream valley that meandered across what is now east-central
Illinois. Meltwater from the glaciers approaching from the north inundated
the valley with sand and gravel, until the region was overrun by glaciers.
When the glaciers finally retreated some 13,000 years ago, the sand
and gravel filling the former stream valley had been buried beneath
hundreds of feet of clay-rich materials that had been deposited by the
glaciers. The porous, water-saturated sand and gravel that makes up
the aquifer was effectively trapped on the bottom and sides by the bedrock,
and on the top by the cap of nearly water-tight, clay-rich glacial till.
The
only known location where water enters the aquifer at a relatively rapid
rate is in Champaign County, where glaciers deposited thin layers of sand
and gravel within the overlying clay. Still, water movement within the
aquifer is slow. Rain and snow that falls on the surface in Champaign
County begins a roughly 3,000-year journey downwards to the Mahomet Aquifer,
traveling at an average rate of less than an inch a year. Once it reaches
the aquifer, it travels laterally in every compass direction but south.
After about 7,000 years, water that journeyed westward seeps into the
Illinois River along the river bottom near Havana, Illinois. As
it turned out, the aquifer the company had tapped was actually the Glasford
Aquifer, a smaller but similar sand and gravel reservoir that overlies
the Mahomet Aquifer. In the 1940s, deeper wells drilled by the Illinois
State Geological Survey (ISGS) revealed the seemingly limitless water
resources of the Mahomet Aquifer. Wells were yielding water at a rate
of 3,000 gallons a minute. Aquifers are like sponges.
Although you can't see the water, it is there, filling the spaces between
particles. Materials made up of particles that are tinier than sand, such
as clay, hold water molecules tightly

Aquifers
are like sponges. Although you can't see the water, it is there, filling
the spaces between particles. Materials made up of particles that are
tinier than sand, such as clay, hold water molecules tightly within their
many pores, making the water difficult to extract. Aquifers made up of
relatively large particles, like sand and gravel, create sizable voids
between the particles so that water has plenty of space to fill and can
flow easily. The Mahomet Aquifer is made up of these latter materials.

Protecting the Aquifer

The notion of protecting
groundwater is as recent as the 1960s. Until then, aquifers had been largely
out of sight and out of mind for most people. What's more, data about
these resources--even their size and location--were scant. Surface-water
resources could be mapped from a few good aerial photos. Documenting the
depth and breadth of water locked within the subsurface required geologists
to drill wells and extrapolate from the data collected at these scattered
points and with other data about the geologic structures. More sophisticated
technologies are providing more data at lower cost, but geologic mapping
in the flat, glaciated terrain of Illinois is still a labor-intensive,
costly endeavor. With limited data, though, the potential for error is
great; until the 1980s, scientists believed that the Mahomet Aquifer was
part of a single system of aquifers called the Teays-Mahomet system, which
stretched all the way to West Virginia. Protecting
groundwater resources from possible contamination has yet to capture the
public's imagination in much of Illinois; however, it is a major concern
for public health agencies. Nearly half of the state's, for that matter
the nation's, population depends on groundwater for its drinking water;
in rural areas, that figure climbs to more than 90%. Deep aquifers, like the Mahomet
Aquifer, are not immune from contamination, but they are better shielded
than most. Water quality problems that now exist are naturally occurring.
For instance, water pumped from the Mahomet Aquifer is "hard" due to dissolved
minerals, especially calcite and dolomite. They pose no health risk, but
the minerals cause a scaly buildup in pots and water pipes if they are
not removed with commercially available softening systems. The iron that
is abundant in Mahomet Aquifer water is also harmless, but it is a nuisance
because it discolors ceramic fixtures if it is untreated. One naturally
occurring contaminant that may pose a health risk in some areas is arsenic.
It appears to

Clay-rich
tills tens to hundreds of feet thick were depositied beneath the
glaciers of the Pleistocene Epoch (top). The result was the entrapment
of the sand and gravel bodies between bedrock below and clay-rich
till above--perfect conditions for the formation of aquifers (bottom). Zoom in to improve image clarity (try the right mouse button).